Electrogalvanized steel sheet, welding-plated steel sheet, and steel sheets plated with various alloys are produced and extensively used in applications such as electrical appliances, automobiles, and building materials. Under these circumstances, there is a growing desire in recent years especially for surface-treated steel sheets having excellent corrosion resistance, and the demand for such steel sheets tends to increase.
In the automobile industry, the demand for surface-treated steel sheets with excellent corrosion resistance is strong from the standpoints of safety and economy because automotive bodies are exposed to severe corrosive environments, as seen in the corrosion caused by rock salt scattered on roads for the purpose of antifreezing in the North America and other regions and the corrosion caused by acid rain attributable to smokes emitted from industrial areas.
Many improvements have conventionally been made in order to obtain the required corrosion resistance. Developed as such steel sheets with improved corrosion resistance were precoated steel sheets obtained by applying a special resin on the chromate material of galvanized steel sheet. Some of the precoated steel sheets are on the market.
In these precoated steel sheets, improved corrosion resistance is attained by the coated film formed by applying a special resin on the sheet steel in an amount of 0.3 to 5 g/m.sup.2.
However, even the precoated steel sheets do not always have sufficient corrosion resistance in a severe corrosive environment. Increasing the coated film thickness in order to ensure corrosion resistance has resulted in a precoated steel sheet which has poor suitability for welding and is virtually unusable for automobiles.
As techniques for further improving the above-described corrosion resistance by improving the anticorrosive properties of a coated film formed in the production of precoated steel sheets, the following have, for example, been disclosed: a method of conducting a treatment with an aqueous solution obtained by adding a colloid (sol) of a silica or the like to an aqueous dispersion of an ethylene-acrylic acid copolymer resin or other resin (JP-A-63-123472); and a method in which a water-based composition obtained by adding a water-dispersible silica and a water-dispersible chromium compound to a dispersion of a copolymer of an .alpha.-olefin-.alpha., .beta.-unsaturated ethylenically carboxylic acid is applied to form an undercoat film, on which a topcoat film is formed (JP-A-3-131371). (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
The above two methods each is characterized in that an additive such as a silica is incorporated into a conventional organic resin for the purpose of improving its anticorrosive properties. The silica colloid to be thus used as an additive is a water-dispersible silica or colloidal silica (both of which are an aqueous colloid of a wet (precipitated) silica). These aqueous colloids of wet silica have already been put to the market by several companies, and examples of the commercial products include various grades of SNOWTEX (trade name) manufactured by Nissan Chemical Industries, Ltd., Japan, and various grades of ADELITE (trade name) manufactured by Asahi Denka Kogyo K. K., Japan. These wet (precipitated) silica colloids are produced by a colloidizing technique in which particle size reduction is conducted by cleavage at siloxane bonds in an aqueous solution having a pH of 9 or higher or by the same cleavage in an aqueous solution having a pH of 2 or lower. Most of the colloidal particles thus obtained usually have particle diameters of from 20 to 60 nm and the colloidal solutions are mostly alkaline, with their pH's being from 8 to 12.
That is, in the production of the colloidal silica described above, a wet (precipitated) silica is used and sodium hydroxide (for alkaline colloid production) or acetic acid (for acid colloid production) is added in order to colloidize the silica. Because of this, the wet (precipitated) silica has a low purity (i.e., contains a large amount of impurities such as Na.sup.+, Fe.sup.3+, and Cl.sup.-) and comes to have an even lower purity due to the addition of other ions during colloid production (usually contains silicate ions and the like in a large amount). Such low-purity silica colloids have no cation-exchange ability and are still insufficient for use as an additive to be incorporated in coating compositions for imparting high anticorrosive properties to coated films.
The example using the wet silica is shown in the comparative example of Example 4 as comparison.
On the other hand, a fumed silica as a high-purity silica has been put to the market by Nippon Aerosil K. K., Japan, and others under the trade name of AEROSIL. This fumed silica is an amorphous compound which has a network structure consisting of chains built up of tetrahedral (SiO.sub.2).sub.n arrangements and which has an extremely low impurity content because it has been obtained by hydrolysis at a high temperature of about 1,000.degree. C. (HCl is completely removed from the system because it vaporizes at the high temperature). The fumed silica, therefore, is a material suitable for use as a silica colloid to be added to coating compositions for the purpose of imparting high anticorrosive properties, as compared with the wet (precipitated) silicas described above.
However, the fumed silica, which immediately after production is in the form of unaggregated primary particles (with particle diameters of about 5 to 12 nm), undergoes aggregation attributable to hydrogen bonds formed by water adsorption because it is a hydrophilic silica (SiO.sub.2).sub.n and has the property of readily adsorbing water. The aggregated particles then relatively readily turn into an amorphous network structure having a silanol group density of from 3 to 6 per nm.sup.2 Because of the presence of silanol groups, the fumed silica in an aqueous solution has a chain structure consisting of primary particles linked together into network form. Illustratively stated, the fumed silica (e.g., AEROSIL (trade name) mentioned above) is an acid pigment an aqueous solution of which has a pH of about 4.1 to 5.6, and this acidity is attributable to protons generated from the silanol groups (SiOH).sub.n. It is, however, thought from the silanol group density and the pH that the hydrogen atoms of only a small proportion of the silanol groups have turned into protons, with most of the silanol groups remaining unionized. This indicates that the unionized silanol groups which are present in a large number on the surfaces of the primary particles undergo a silanol-silanol interaction (formation of hydrogen bonds) to bond to one another and, as a result, the primary particles are linked to form chains of a network structure. Linking through a water molecule is also possible, and this linking mode is also based on a hydrogen bond.
Since these hydrogen bonds of the fumed silica in an aqueous solution have a lower bond energy than ionic bonds and covalent bonds, the hydrogen bonds can be cut by applying a physical energy in such a degree as disper stirring (e.g., at 2,000 rpm for 30 minutes) to the solution and, as a result, the network structure can be converted to aggregated particles (colloidal particles). However, the aggregated particles, upon standing, readily turn into larger aggregates or return to particles of a network structure, i.e., become coarser. Because of this, it has been extremely difficult to add the fumed silica in a colloidal state to a coating composition and stabilize the colloidal silica added.
The example using the fumed silica is shown in the comparative example of Example 2 as comparison.
In addition to the high rust resistance described above, a precoated steel sheet of the kind described hereinabove is required to further have suitability for electrodeposition and press processing, spot weldability, corrosion resistance after electrodeposition, and secondary adhesion.
In order to meet these requirements, improvements have conventionally been made on coating compositions for forming the coated films of precoated steel sheets. For example, JP-A-62-73938, JP-A-62-283161, and EP 307996A (corresponding to JP-A-64-65179) disclose a technique for imparting electrodeposition suitability by incorporating the above-mentioned fumed silica. However, a silica content of 27.5% or higher is necessary for obtaining electrodeposition suitability on a virtually required level and such a high silica content not only results in impaired suitability for press processing and spot welding, but causes unevenness of treatment in a degreasing step due to the dissolution or falling of silica particles during the step. Further, JP-A-62-283161 discloses a technique for imparting electrodeposition suitability by incorporating a hydrophilic resin (e.g., a polyamide resin). However, this technique is defective in that the content of the hydrophilic resin should be from 5 to 40% in order to obtain the required electrodeposition suitability, resulting in impaired anticorrosive properties, and that the solvent to be used is limited (to dibenzyl alcohol), which is disadvantageous from economical and other standpoints. Also disclosed is a technique of incorporating an electrically conductive pigment or a metal. This technique, however, is defective in that since relatively coarse particles such as a metal powder, zinc dust, or metal carbide are incorporated as the additive, a coated film formed from the coating composition not only makes an electrodeposited film formed thereon have considerable surface roughness, i.e., poor surface smoothness, but also has insufficient suitability for press forming.
As described above, the conventional techniques are still far from attaining well balanced coated film performances. If the incorporation of such a silica, hydrophilic resin, electrically conductive pigment, etc., even in relatively small amounts is effective in obtaining electrodeposition suitability, this method is more practical because various techniques can be employed more easily, such as alteration of base resins, incorporation of hardeners and rust preventives, and selection of such additives from wider ranges.